1. Field of the Invention
The present invention relates generally to an anti-skid brake control system for automotive vehicle, which controls braking pressure in order to maintain wheel slippage at an optimum level for optimal vehicular braking performance. More specifically, the invention relates to a derivation of wheel acceleration at selectively variable timing depending upon the vehicle driving condition so that precision level and response characteristics of derivation of wheel acceleration data may be suitably adapted to a requirement which can be varied depending upon the vehicle driving condition.
2. Description of the Background Art
Recently, various constructions and operations of anti-skid vehicular brake control systems have been proposed for improving deceleration performance of an automotive vehicle to assure safety in braking. In the anti-skid brake control system, a skid control cycle, in which braking pressure is controlled or adjusted so as to avoid excessive wheels slippage due to locking or skidding of wheels, is initiated in response to a wheel deceleration (negative value of acceleration). During anti-skid brake control, braking pressure is adjusted over one or more skid control cycles for maintaining braking pressure in the vicinity of lock pressure to maintain wheel slippage at an optimal level, i.e. 10 to 20%.
In such anti-skid brake control system, it is desirable to timely initiate a skid control cycle for obtaining the optimum brake control performance. For this, a high precision level is required in derivation of wheel acceleration. Higher precision level of wheel acceleration data may permit setting of wheel deceleration threshold which is set for detecting initiation timing of skid cycle when wheel acceleration is decreased. Setting of the wheel deceleration threshold at lower level may delay initiation timing of skid control cycle and thus permit increasing the braking pressure at a higher level. This is clearly desirable for improving vehicular braking performance. However, in usual case, due to error contained in the wheel acceleration data, the wheel deceleration threshold cannot be set at satisfactorily low level.
As will be appreciated, higher precision in derivation of wheel acceleration data can be obtained by expanding period for sampling wheel speed data. For examples, the following United States Patents which have been all assigned to the common assignee disclose relevant technologies in derivation of wheel acceleration data with acceptable precision level for using in the anti-skid brake control.
U.S. Pat. No. 4,683,537, issued on July 28, 1987
On the other hand, in the U.S. Pat. No. 4,683,537 discloses another technology for performing anti-skid brake control. In the invention disclosed in this United States Patent, the peak value of the wheel acceleration data while the wheel speed resumes toward a vehicle body speed and overruns the vehicle body speed, is detected. Based on the detected peak value of the wheel acceleration data during acceleration mode, road surface condition, i.e. road/tire friction level, is detected for adjusting operation in brake pressure increasing mode (hereafter APPLICATION mode) in the succeeding skid control cycle. For this purpose, it is essential to obtain wheel acceleration data with a minimum lag time. Therefore, when the wheel speed data sampling period is set at relatively long period, it necessarily cause; increasing of lag time in detection of the peak value of the wheel acceleration data. In view of required response characteristics in detection of the peak value of the wheel acceleration data, it is desirable to minimize the wheel speed data sampling period.
Therefore, it is desired to achieve both high precision in deriving wheel acceleration data for detecting initiation timing of skid control cycle and higher response characteristics in derivation of the wheel acceleration data for detecting the peak value of the wheel acceleration data.